A valve used for liquid often has a gas lock that prevents the valve from dripping when a closure element closes the valve.
In such a filling valve, the closure element is normally attached to a tappet that moves up and down. To open and close the valve, one normally moves the closure element up and down.
Within the filling valve is a valve chamber. The valve tappet and the closure element are both in this valve chamber. At its bottom end, the valve chamber has an opening for allowing filling product to leave it. In operation, the valve chamber is completely filled with filling product.
To close the filling valve, one moves the closure element from its open position to its closed position. This involves moving the valve tappet down. This means that more of the tappet's length will be inside the valve chamber. Naturally, this leaves less room for liquid in the chamber.
As noted above, the valve chamber is completely filled with filling product. This means that when the valve tappet moves down, it displaces liquid. This liquid has to go somewhere. Typically, this displaced liquid cause the gas lock to fail. As a result, the valve drips. In some cases, the displacement also causes a noticeable surge in pressure. This leads to even more dripping.